Temperature and Particle Motion
Temperature is a measure of the average kinetic energy of the particles in a substance: the faster the particles move, the higher the temperature. In brief: heating a substance makes its particles move faster, and the Kelvin scale starts at absolute zero (0 K = −273.15 °C), where particle motion is at its minimum.
Temperature is often confused with heat. Temperature describes how hot something is, and heat is energy transferred because of a temperature difference. The particle model clears up the difference. This article explains temperature in terms of particles, introduces the Kelvin scale and absolute zero, compares temperature and heat in a table, includes conversion calculations and tells the story of how temperature scales were established.
What does temperature mean for particles?
In the particle model, particles are always moving. In a hot substance they move with more kinetic energy on average than in a cold one. For gases, the average kinetic energy is directly proportional to the temperature in kelvin. If the absolute temperature doubles, the average kinetic energy of the particles doubles.
In a solid, the particles vibrate more strongly at higher temperatures. In a liquid, they move faster and slide past each other more easily, which is why hot liquids are often less viscous than cold ones. Temperature does not depend on the number of particles: a cup of tea and a large pot of tea at 80 °C have the same temperature, though the pot holds much more thermal energy.
What is the difference between temperature and heat?
| Feature | Temperature | Heat |
|---|---|---|
| What it measures | Average kinetic energy per particle | Energy transferred due to a temperature difference |
| Unit | °C or K | joule (J) |
| Instrument | Thermometer | Calorimeter |
| Depends on amount of substance? | No | Yes |
| Direction | Not applicable | From hot to cold |
Heat flows from the hotter object to the colder one until both reach the same temperature, a state called thermal equilibrium.
What is the Kelvin scale?
The Kelvin scale is the SI scale of temperature. Its zero, absolute zero, is the lowest possible temperature, at which particles have minimum energy. One kelvin has the same size as one degree Celsius, so a temperature difference is the same in both scales. The relationship is:
T(K) = t(°C) + 273.15
Kelvin temperatures have no degree symbol and are written as, for example, 300 K. In calculations with gases the temperature must be in kelvin, since a Celsius value could be zero or negative, but the energy of the particles cannot be.
Important temperatures in three scales
| Event | Celsius (°C) | Kelvin (K) | Fahrenheit (°F) |
|---|---|---|---|
| Absolute zero | −273.15 | 0 | −459.67 |
| Boiling of liquid nitrogen | −196 | 77 | −321 |
| Freezing of water | 0 | 273.15 | 32 |
| Room temperature (typical) | 20 | 293.15 | 68 |
| Human body (normal) | 37 | 310.15 | 98.6 |
| Boiling of water | 100 | 373.15 | 212 |
Worked example: converting and comparing
- Celsius to kelvin. 25 °C + 273.15 = 298.15 K, usually rounded to 298 K.
- Kelvin to Celsius. 350 K − 273.15 = 76.85 °C.
- Celsius to Fahrenheit. °F = °C × 9/5 + 32. For 37 °C: 37 × 1.8 + 32 = 98.6 °F.
- Comparing kinetic energies. Gas particles at 600 K compared with 300 K: the average kinetic energy is proportional to T, so the ratio is 600 ÷ 300 = 2. The energy doubles.
- Be careful with Celsius. A gas at 40 °C compared with one at 20 °C: the Kelvin values are 313.15 K and 293.15 K. The ratio is 313.15 ÷ 293.15 = 1.07, so the energy is only 7% greater, not double.
Why do substances expand when heated?
When the temperature rises, particles vibrate or move with larger amplitude, and the average distance between them grows. The substance therefore expands. Gases expand most, liquids less and solids least. Thermometers use this effect. The liquid in a thermometer expands up the narrow tube as the temperature increases. Engineers leave expansion gaps in bridges and railway tracks for the same reason.
How were temperature scales developed?
In 1714 Daniel Gabriel Fahrenheit made mercury thermometers and a scale based on fixed points. In 1742 Anders Celsius proposed a scale with 100 divisions between the freezing and boiling points of water, which was originally written the other way round and later inverted. In 1848 William Thomson, later Lord Kelvin, proposed an absolute temperature scale based on the idea of a lowest possible temperature. The kelvin was defined in the 2019 revision of the SI by fixing the value of the Boltzmann constant.
Which mistakes are common?
Mistake 1: Using temperature and heat as synonyms
Temperature is a measure of average particle energy. Heat is energy transferred.
Mistake 2: Using Celsius in gas calculations
Convert to kelvin first.
Mistake 3: Believing that doubling the Celsius temperature doubles the energy
Only the Kelvin temperature is proportional to the average kinetic energy.
Mistake 4: Saying that cold is a substance that flows
Heat flows from hot to cold. Cold is the lack of thermal energy and does not flow.